Television image projection tube



M. VON ARDENNE TELEVISION IMAGE PROJECTION TUBE Filed June 6, 1940 Match17, 1942.

G I I I I I V INVENTOR MAIVFRED VON ARDEN/V5 ATTORNEY v Patented Mar.17, 1942' OFFICE TELEVISION IMAGE PROJECTION TUBE Manfred von Ardenne,Berlin-Lichterfelde, Germany Application June 6, 1940, Serial No.339,052 In Germany December 6, 1938 3 Claims.

This invention relates to television image projection systems whereinoptical means are provided for projecting a television image upon ascreen. Television receiving tubes for use in such a system are shownand described in applications Serial #292,017, filed August 26, 1939,and in Serial #306,610, filed November 29, 1939, as well as inother'applications filed by me.

In experiments carried out with arrangements such as shown and describedin the application Serial #292,017 and more especially with those of theapplication Serial #306,610, polarization charges were encountered atthe separating surface between the crystal layer and auxiliary layer.These charges appear gradually and, in accordance with the proportion ofthe conductivity of the auxiliary layer and of the crystal, they causedeither a slow disappearance of an originally intense luminescence at aconstant storage potential, or else a longer sustaining of aluminescence following the extinction of the storage potential. Thesecharges, therefore, cause in part either profound falsiflcations of thetone values of the image having constant brightness value, or in imageswhere the brightness varies they cause trailing disturbances such as areproduced, for instance, by fluorescent screens having the property ofafter slow. The elimination of these disturbances is an importantproblem when employing crystal screens with auxiliary layers having alow secondary emissivity.

Th present invention may best be understood by referring to the drawingwherein like reference characters represent like parts, and wherem:

Figure 1 shows one form of the present invention.

Figure 2 shows an enlarged view of one form of the crystal platestructure or target electrode.

In Figure l of the drawing is shown a cathode ray tube 8 in which ispositioned a gun structure It for producing a narrow focused beam ofelectrons. The intensity of the beam of electrons is determined by thepotential of the control electrode l2. Als positioned within the tube isa target electrode which comprises a support plate 22, a layer ofcrystals 20, and an auxiliary layer or film of transparent material 26which has a predetermined (preferably low) secondary electron emissivecharacteristic. Positioned in front of the target electrode is anelectron accelerating screen 52 for producing rapid acceleration of theelectrons which constitute the beam in the vicinity of the targetelectrode. The cathode ray beam H which is generated by the gunstructure I0, is caused to be deflected over the target electrodeincluding the crystal plate by means of the horizontal and verticaldeflecting means l6 and I8, respectively.

A source oi. light, 36, is provided, and the light which originates fromthe source is directed toward the receiving tube by means of thereflector 34. The light is passed through a lens 38 and a polarizingscreen 46 before being projected upon the crystal plate 20. Positionedon the other side of the receiving tube is a still further polarizingscreen 48 and lens system 40, in order to focus the produced opticalimage upon a viewing screen 44 in an enlarged manner.

As described in the above mentioned application, Serial #292,017, theamount of light which is permitted to pass through the crystal plate isdetermined by the charge which the various elements of the crystal platehave assumed in accordance with the intensity of the scanning cathoderay beam, and the light rays so modulated are finally projected upon thescreen 44 where the enlarged and intensified television image isreproduced.

One form of the crystal plate or target electrode is shown in Figure 2,and as stated above, this includes the support 22 upon whichispositioned a layer of crystals 20 which may be prepared and arrangedin the manner suggested in application Serial #307,573, filed December5, 1939. The layer of crystals 20 may be composed of Segnette salts orthe crystal layer may be formed of sulphide of zinc or zinc blende. Theauxiliary layer 26, which is preferably light transparent, and which hasa predetermined secondary electron emissive characteristic, is attachedto the layer of crystals 20 by any appropriate means, such as, forexample, by means of a layer of cement or adhesive material 24. Whencement is used it should Preferably be clear or transparent and shouldbe a substance that is stable in vacuum, such as, for example,waterglass. In an alternative form the layer of cement may be omittedand the auxiliary layer may be placed on the layer of crystals while ina liquid state and later permitted to become hardened. It is necessarythat very intimate contact be maintained between the auxiliary layer 26and the crystal layer 20, as will be explained later. The thickness ofthe auxiliary layer 26, as well as its conductivity and dielectricconstant, are chosen in accordance with the correspondingcharacteristics of the particular crystals used in the layer 20, as willbe more fully explained. As suggested in application Serial No. 306,610,the layer 26 must possess high insulating qualities and may consist ofglass or a vitreous layer. Alternatively, mica platelets or scales ofsimilarly clear transparent substances may be used.

In order to eliminate the disturbances caused by polarization chargesvarious ways are open. The most obvious measure resides in preventingthe appearance of true charges at the separating surface between theauxiliary layer 26 and the crystal layer 20. A calculation carriedthrough in this respect shows that true charges cannot occur if theproportion between the conductivity and the dielectric constant of heauxiliary layer 28 is the same as that between the conductivity of the.crystal and its dielectric constant (in the direction of the field).Polarization charges hence ditions the conductivity within limits (forinh stance, the conductivity'of various types of glass, i. e., thespecial Schott glass having a high conductivity), the control and choiceof the conductivity of the auxiliary layer to conform to the aboverequirement involves no principal difllculties.

' It is obvious that care must beataken that vacuum interspaces betweenthe crystal and auxiliary layer caused by insufilcient contacting do notexist. Very intimate contact between the 1 two layers may be attainedeither by employing a cement 28 previously referred to (see ApplicationSerial #306,610) but which would have to gomply with the aforesaidcondition, or else by applying the auxiliary layer in a heated liquidstate, to1 the crystal layer and causing a Joining of both. There couldalso be used as auxiliary layer a layer of cementing substance properwhich fulfills the aforesaid requirement and which has at the same timethe property of low secondary emissivityjand optical clarity combined.

Another entirely diiferent way of eliminating the charge disturbancesconsists of dimensioning the layer thicknesses and conductivities insuch a manner that the polarization charges can pass ofi at suchrapidity that the trailing disturbance referred to above will besuppressed in images where the light content is variable or where motionis present. This signifies that the polarization charges must be passedoff within about second, At this procedure there exists next the danger;that the useful or desired charges will be weakened to a critical pointby the leakage during the time period extending from thebeginsumciently' thin layer fulfilling the requirement of suitableconductivity it can be readily achieved that during the period ofextinction the polarization charge together with the useful chargewillbe passed through the auxiliary layer without detriment to the sharpnessof the contours of the image on account of lateral leakage of the usefulcharges. This is explained by the fact that the conductivity of anauxiliary layer as so dimensioned differs by several order values in thevarious-directions thereby accomplishing the de:

sired result.

The last mentioned procedure increases in eftelevision standard withscanning according to the line interlace method. The time period between the storing and extinction of the individual image elements wouldin this case be of the order of 1/60 second so that the conditions ofthe conthe thinner the auxiliary layer can be chosen with respect to thepenetration depth of the scanning cathode'ray beam of electrons. At voltvelocities of the order of 10,000 volts and with the layer materials tobe considered the depth of penetration is of the order of some 10- mm.If the storing device can be operated with a plate potential of 10,000volts, i. e., if auxiliary layers having such a. small thickness can beemployed, thelast mentioned method affords an especially suitable andsafe solution of the present problem since in this method an absolutelyaccurate matching of the conductivities is not required but.

solely a proper dimensioning of the conductivities in respect to ordervalues.

I claim:

l. A cathode ray tube for a television receiving system having meanstherein adapted for generating a beam of electrons, a target area, meansfor deflecting the beam of electrons to cause the same to scan thetarget area, said target area comprising transparent support base,

alayer of crystals of substantially uniform thickness positioned on thesupport base, and a translayer being substantially equal to thecorrespond ing characteristics of the crystal layer.

2. A cathode ray tube for a television receiving system having meanstherein adapted for generating a beam of electrons, a target area, meansfor deflecting the beam of electrons to cause the same to scan thetarget area, said target area comprising a transparent support base, alayer of crystals of substantially uniform thickness positioned on thesupport base, and a translucent film or auxiliary layer of materialpositioned upon the crystal layer, said .auxiliary layer having apredetermined secondary electron emissive charductivity, which have thepolarization charges leak off within second still do not produce acritical weakening of the useful charge in the substantially shorterperiod of storing.

A special case' of the above discussed second -measure resides'in thatat the given properties of the crystal plate 20 the auxiliary layer 26is com- .product of the conductivity of the crystal layer acteristi-c,the product of the conductivity of the auxiliary layer times thedielectric constant of the crystal layer' being substantially equal tothe in a direction normal to its surface times the ch electric constantof the auxiliary layer.

posed of a material having a much better conductivity (conductivitysuperior to that of the crystal) but it should be produced with such a3. A target electrode for a television receiving tube comprising atransparent support base, a layer ofcrystals of substantially uniformthickness positioned on said support base, and an auxiliary film oftranslucent material positioned upon the crystal layer, the ratio of theconduc-' tivity of the auxiliary film to" the dielectric con- 'stantofthe auxiliary film being substantially equal to the ratio ofconductivity of the crystal layerin a direction through the layer to thedielectric constant of th "crystal layer,

" ED VON "ARDENNE.

